In a digital radio communication system a base station transmits a signal at a transmission rate to a remote unit through a radio channel having channel characteristics, such as an attenuation. The signal is transmitted using transmission parameters, such as a modulation level and a coding rate. The transmission rate depends on the transmission parameters. The transmission parameters are constrained by an acceptable bit error rate and by a signal to interference ratio of the signal, the latter varying in time with the channel characteristics. The communication system can use adaptive modulation to adjust the transmission parameters to accommodate changes in channel characteristics over time. If a change in channel characteristics results in a lower signal to interference ratio, the modulation level must be reduced (for example, from 16-QAM to QPSK) or the coding rate must be improved (for example, from ¾ to ⅔) in order to maintain the acceptable bit error rate, albeit at a lower transmission rate. If a change in channel characteristics results in a higher signal to interference ratio, the base station can increase the modulation level or decrease the coding rate in order to obtain a higher transmission rate.
In a communication system that implements adaptive modulation, the base station and the remote unit must be synchronized with respect to the transmission parameters. In current communication systems the remote unit determines a channel quality when the remote unit receives a frame of data. The remote unit may estimate, for example, the signal to interference ratio of the channel. The remote unit sends a signal back to the base station reporting the channel quality. Using the channel quality report received from the remote unit, the base station calculates a set of optimum transmission parameters which the base station will use in its next transmission of data. However, the base station must first send the set of new optimum transmission parameters to the remote unit using the previous transmission parameters. The remote unit receives the set of new optimum transmission parameters, interpreting the signal using the previous transmission parameters. The remote unit then decodes subsequent frames of data using the new optimum transmission parameters.
In communication systems that make use of multiple antennae for transmission and reception, the transmission parameters may include adaptive antenna and coding parameters. For example, some “smart antenna” systems may adaptively adjust their directional patterns towards the remote units. An outline of such systems may be found in the paper by J. H. Winters, “Smart Antennas for Wireless Systems”, IEEE Pers. Commun., vol. 5, no. 1, Febuary 1998, pp. 23-27, which is incorporated by reference herein. Similarly, the radio system may make use of the multiple communication channels that exist between transmitters and receivers with multiple antennae. In this case, the transmission parameters include both space (across multiple antennae) and time (different time of transmission) aspects that adapt the transmissions to the multiple propagation environment. An outline of such systems may be found in the paper by A. J. Paulraj and B. C. Ng, “Space-time Modems for Wireless Personal Communications”, IEEE Pers. Commun., vol. 5, no. 1, Febuary 1998, pp. 36-48, which is incorporated by reference herein.
In communication systems employing many sub-carriers, such as those that employ Orthogonal Frequency Division Multiplexing (OFDM systems), the channel quality may vary with the frequency of each sub-carrier. OFDM systems can use 1000 sub-carriers, and transmission of information describing the channel quality and a set of optimum transmission parameters for each sub-carrier would require significant overhead, reducing the efficiency of the communication system. In current methods, the signal to interference ratio is averaged over all sub-carriers so that only one signal to interference ratio is reported to the base station and only one set of new optimum transmission parameters is transmitted to the remote unit. In this method, the single set of new optimum transmission parameters results in an unnecessarily low transmission rate for individual sub-carriers whose signal to interference ratio is higher than the average signal to interference ratio reported by the receiver.